In recent years, a medicine to inhibit glucose-reabsorption by Na+-glucose cotransporters (SGLT) in the intestinal tract and kidney (a Na+-glucose cotransporter inhibitor) has been demanded as an antidiabetic agent to rapidly normalize hyperglycemia and improve the energy balance in the body. Such a Na+-glucose cotransporter inhibitor has been expected as an excellent agent for treating or preventing various diabetes-related diseases such as insulin-dependent diabetes (type 1 diabetes) and insulin-independent diabetes (type 2 diabetes), as well as insulin-resistant diseases and obesity.
As compounds used for the Na+-glucose cotransporter inhibitor, phloridzin described in Welch, C. A. et al. (J. Natr., 1989, 119(11) 1698) and synthetic O-glycoside derivatives described in Hongu, M. et al. (Chem. Pharm. Bull., 1998, 46(1) 22) and JP-A-11-21243 are known, for example. These compounds are reported to discharge excess blood glucose into urine and reduce blood glucose level by inhibiting glucose-reabsorption by Na+-glucose cotransporters in the intestinal tract or in the kidney.
However, because any of these compounds is an O-glycoside derivative comprising an O-glucoside bond formed between glucose and an aglycon moiety, it has a problem that the inhibition effect is reduced due to hydrolysis of O-glucoside bond by glucosidase or the like in the small intestine when orally absorbed.
In addition, in the case of phloridin, phloretin, which is an aglycon moiety of phloridin, is known as a strong inhibitor for a facilitated diffusion-type glucose transporter. For example, it is reported that the cerebral glucose concentration decreases when phloretin is administered to the vein of a rat (e.g. Stroke, 1983, 14, 388). Phloretin is also known as an inhibitor of a vitamin C transporter (Wang, Y. et al., Biochem. Biophys. Res. Commun., 2000, 267, 488-494).
Therefore, an attempt has been made to use a C-glycoside prepared by converting oxygen in the glucoside bond of the O-glycoside to carbon as the Na+-glucose cotransporter inhibitor.
For example, JP-A-2001-288178 (Patent Document 1) describes that compounds of the following formula are effective in inhibiting Na+-glucose cotransporters and are useful as a treating agent or preventing agent for diabetes and a hypoglycemic agent.
wherein R1 represents H, OH, lower alkyl group, —O-lower alkyl group, or the like; R2 represents H, —COO-lower alkyl group, or the like; R5 represents —CH2OH, —CH2OCOO-lower alkyl group, or the like; A1 represents pyridine, furan, thiophene, quinoline, indole, or the like; n is 0, 1, 2, or 3, and m is 0 or 1 (See Patent Document 1 for further details on the symbols of the above formula).
In addition, the pamphlet of WO 01/27128 (Patent Document 2) describes that a compound of the following formula can be used as the Na+-glucose cotransporter inhibitor to treat obesity or type 2 diabetes.
wherein R1, R2, and R2a individually represent a hydrogen atom, OH, OR5, alkyl, CF3, OCHF2, OCF3, or the like; R3 and R4 individually represent a hydrogen atom, OH, OR5a, —O-aryl, —O—CH2-aryl, alkyl, cycloalkyl, CF3, or the like; A represents O, S, NH, or (CH2)n, and n is 0, 1, 2, or 3 (See Patent Document 2 for further details on the symbols of the above formula).
As explained above, the C-glycoside derivatives are useful to a certain extent for treating diabetes due to the effect of inhibiting a Na+-glucose cotransporter. However, due to the recent rise in incidence of diabetes which is a lifestyle-related disease and could even be called one of the most popular diseases in Japan, compounds having a chemical structure different from that of known compounds and showing the effect of inhibiting Na+-glucose cotransporters more rapidly and more significantly have been increasingly desired for the clinical practice of diabetes treatment or the like.